The present invention relates to a tomograph utilizing the phenomenon of nuclear magnetic resonance (hereinbelow, termed `NMR`), which is used for medical diagnoses.
The constituents of an NMR imaging apparatus include a static magnetic field, a gradient magnetic field, an rf magnetic field, a detection part, etc. Since the positions of spins are determined by the combination of the static field and the gradient field, the spatial uniformities of these magnetic fields are required.
The inhomogeneity of a static field distribution brings an image phase distortion, spatial distortion and density distortion. On the other hand, the inhomogeneity of a gradient field distribution brings an image spatial distortion and density distortion. Accordingly, it is necessary for enhancing the quality of an NMR image to precisely measure the static and gradient field distributions and to correct the image on the basis of the measured values.
Heretofore, techniques in which only the distribution of the static field is measured have been proposed. There are Kawanaka's method and Sekihara's method (Kawanaka et al., "METHOD OF AUTOMATICALLY CORRECTING IMAGE DISTORTIONS ASCRIBABLE TO NON-UNIFORMITIES OF STATIC FIELD IN NMR IMAGING," Transactions of IECE, '85/3, Vol. J68-D; Sekihara et al., "NEW METHOD OF MEASURING A STATIC FIELD DISTRIBUTION USING NMR IMAGING," JAMIT, Vol. 2S, No. 1). The Kawanaka method is a technique relating to the spatial distortion of an image, while the Sekihara method is the technique with note taken of the phase distortion of an image. The prior-art methods, however, have not been sufficient in precision because the inhomogeneity of the gradient field distribution is neglected though it is also a factor for causing the spatial distribution.
In measuring static fields, methods directly employing instruments are common, but they have problems in the measuring period of time and the precision. Recently, there have been proposed techniques in which the distribution of the static field is measured using an imaging apparatus itself and on the basis of data obtained by imaging a known object.
Typical is a method by Sekihara, "NMR IMAGING FOR MAGNETS WITH LARGE NON-UNIFORMITIES." In this method, a uniform phantom is measured using a sequence shown in FIG. 1, and a static field distribution is calculated from the phase information thereof. The method will now be explained more in detail.
The point of difference between the sequence in FIG. 1 and an ordinary sequence lies in the manner of selecting .tau..sub.1 and .tau..sub.2 in the figure. In the ordinary sequence, .tau..sub.1 =.tau..sub.2 is set for measurement, whereas in FIG. 1, .tau..sub.1 .noteq..tau..sub.2 is set for the measurement. The origin of an NMR signal 108 to be measured (the position of a peak P in the figure) can be moved at will by controlling the application period of time .tau..sub.3 of a gradient field 103. The phase .theta. of an image measured according to the sequence of FIG. 1 takes a value which is proportional to the time difference (.tau..sub.1 -.tau..sub.2) between .tau..sub.1 and .tau..sub.2 and the error .epsilon. of the static field distribution. That is, .theta..music-sharp..epsilon.(.tau..sub.1 -.tau..sub.2) holds. Since the value of (.tau..sub.1 -.tau..sub.2) is known, the static field distribution can be obtained from the phase .theta.. In this case, in order to eliminate the influences of a phase distortion attributed to a detecting part and a phase distortion attributed to the fact that (.tau..sub.1 -.tau..sub.2) cannot be accurately controlled, measurement is also performed with the sequence of .tau..sub.1 =.tau..sub.2, and the phase .theta. is conjectured from the resulting difference.
However, the above method, the following disadvantages are involved:
At least two measurement operations are necessary.
For the purpose of eliminating the influences of the phase distortions attributed to the factors other than a static field inhomogeneity, it is necessary to take an image at .tau..sub.1 =.tau..sub.2 and to subtract the phase of the image of .tau..sub.1 =.tau..sub.2 from the phase of an image at .tau..sub.1 .noteq..tau..sub.2. That is, at least two measurement operations are required.
In addition, The dynamic range narrows.
As stated above, the two measurement operations are performed to execute the subtraction of the phases, therefore, the dynamic range narrows. Now, letting .theta..sub.1 denote the phase angle at .tau..sub.1 .noteq..tau..sub.2 and .theta..sub.2 denote the phase angle at .tau..sub.1 =.tau..sub.2, the following conditions need to be satisfied: ##EQU1##
Thus, in order to meet the conditions at all times, the dyanmic range narrows as follows: EQU .vertline..theta..sub.1 .vertline..ltoreq..pi./2 EQU .vertline..theta..sub.2 .vertline..ltoreq..pi./2
This indicates that the setting of .tau..sub.2 for the measurement operations becomes more difficult.